Phase change materials are used in standard bulk silicon technologies to form the memory elements of nonvolatile memory devices. Phase change materials exhibit at least two different states, one being amorphous and the other(s) crystalline. The amorphous state is characterized by the absence of crystallinity or the lack of long range order, as opposed to crystallized states, which are characterized by a long range order. Accordingly, the order in a unit cell, which is repeated a large number of times, is representative of the whole material.
Each memory cell in a nonvolatile memory device may be considered as a variable resistor that reversibly changes between higher and lower resistivity states corresponding to the amorphous state and the crystalline state of the phase change material. The states can be identified because each state can be characterized by a conductivity difference of several orders of magnitude. In these devices, the phase changes of the memory element are performed by direct heating of the phase change material with high programming currents. Conventionally, bipolar transistors are used to deliver high programming currents by directly heating the phase change material. The high current produces direct heating of the phase change material, which can cause the phase change material to degrade over repeated programming operations, thereby reducing memory device performance.
Among the materials of practical use today, most contain tellurium. Of those materials, the most extensively studied material is Ge2Sb2Te5. While the deposition can be conventionally performed by plasma vapor deposition (PVD) techniques such as sputtering, chemical vapor deposition (CVD) and atomic layer deposition (ALD) and related techniques including pulse-CVD, remote plasma CVD, plasma assisted CVD, plasma enhanced ALD), a variety of materials are now being studied in order to overcome the challenges of deposition in complex structures, including those consisting of trenches. The use of Ge(tBu)4, Sb(iPr)3 and Te(iPr)2 has been reported, for instance. The use of such molecules for the deposition of germanium-antimony-tellurium (GST) material raises some difficulties, however. For example, tellurium alkyl or aryls compounds (e.g., Te(iPr)2), are insufficiently thermally stable for a reproducible process. Although there have been significant advancements in the art, there is continuing interest in the design and use of precursor compounds with improved stability.